Risk of subarachnoid haemorrhage in first degree relatives of patients with subarachnoid haemorrhage: follow up study based on national registries in Denmark.

OBJECTIVE To estimate the risk of occurrence of subarachnoid haemorrhage in first degree relatives (parents, siblings, children) of patients with subarachnoid haemorrhage. DESIGN Population based cohort study using data from the Danish National Discharge Registry and the Central Person Registry. SUBJECTS Incident cases of subarachnoid haemorrhage admitted to hospital from 1977 to 1995 (9367 patients) and their first degree relatives (14 781). MAIN OUTCOME MEASURES The incidence rate of subarachnoid haemorrhage was determined for the relatives and compared with that of the entire population, standardised for age, sex, and calendar period. This process was repeated for patients discharged from neurosurgery units, as diagnoses from these wards had high validity (93%). RESULTS 18 patients had a total of 19 first degree relatives with subarachnoid haemorrhage during the study period, corresponding to a standardised incidence ratio of 2.9 (95% confidence interval 1.9 to 4.6). Patients discharged from neurosurgery wards had a higher standardised incidence ratio (4.5, 2.7 to 7.3). CONCLUSIONS First degree relatives of patients with subarachnoid haemorrhage have a threefold to fivefold increased risk of subarachnoid haemorrhage compared with the general population.